Gallium nitride (GaN) films are typically grown using metal-organic chemical vapor deposition (MOCVD) process on one of several available substrates. The historically first and most common substrate material used for growing GaN has been sapphire (hexagonal aluminum oxide) [11]. Although many good gallium-nitride electronic and optoelectronic devices have been grown on sapphire, this crystal is not lattice matched GaN, which results in grown films with relatively high dislocation density. Furthermore, sapphire has low thermal conductivity with respect to GaN and many other substrates used for growth of GaN layers. Consequently, a substantial improvement in device performance is expected with the development of gallium nitride on other substrates that are better lattice-matched substrates and have high thermal conductivity. Other substrates commonly used that have better thermal conductivity than sapphire, but are still not lattice-matched to GaN are silicon (Si), silicon carbide (SiC), and aluminum nitride (AlN). Due to the lattice mismatch the dislocation density of GaN films grown on above mentioned substrates is high—greater than 1E9 cm-2 in some cases. This is a significant disadvantage for the development of high-performance semiconductor devices that require dislocation densities less than 1E6 cm-2 to realize high mobility and/or low non-radiative recombination rates.
Growth of Low Dislocation-Density Gallium Nitride
Growth of GaN crystal on wafers that are not lattice-matched to GaN results in dislocations embedded in the film. These dislocations terminate at the top surface of the grown structure where the electronic or optoelectronic devices are formed, thereby adversely affecting the performance of electronic and optical devices. Dislocation density, measured in number of dislocations per unit area, is an essential property of single-crystal semiconductor surfaces for manufacturing of optical and electronic devices. A dislocation density lower than 1E6 cm-2 is considered low, while dislocation densities of 1E4 cm-2 are considered very low and are presently common in, for example, commercially available gallium arsenide wafers, but can be obtained in gallium nitride as well. Other properties relevant to growth of epitaxial layers are macroscopic defect density, which is typically below 25 cm-2 and full-width half maximum (FWHM) of double-crystal X-ray rocking curves, which for high-quality gallium nitride is below 100 arcsec.
Dislocation densities as low as 1E4 cm-2 can be obtained with GaN layers growing on bulk GaN wafers. High quality electronic and optical devices can then be grown on top of such wafers using MOCVD [2]. The disadvantage of bulk GaN wafers is their high cost [1]: They are grown either in boules [16] (similar to silicon, however, the boules are significantly smaller than those used to grow silicon or gallium arsenide due to process limitations) or as thick epitaxial layers on sapphire substrates where the substrate is later removed. Each wafer of bulk GaN today costs almost one hundred times more than the wafers conventionally used for GaN growth, for example, sapphire. Each device coming from such a wafer hence carries a significant cost of material (the bulk GaN wafer) most of which is actually not being used: Only the top surface of the gallium nitride wafer is used for growing active devices; the vast majority of the wafer volume is used as a carrier and not for its electronic properties—it is thereby wasted.
It is clear that there is a need in the industry to provide layers of bulk-quality gallium nitride with low dislocation density for growth of active devices, but at a cost that is comparable or lower than that of present conventional GaN substrates and preferably at a cost that is comparable to that of silicon. Additionally, a process that would allow the use of thin films of bulk-grown GaN rather than wasting an entire wafer for a single layer of devices would be preferred.
Thermal Management
Thermal management in semiconductor devices and circuits is a critical design element in any manufacturable and cost-effective electronic and optoelectronic product. This is more so the case with high power devices, such as, high-brightness light-emitting diodes, high-power lasers, output-stage electrical signal amplification in microwave devices, and power electronics. The goal of an efficient thermal design is to lower the operating temperature of such electronic or optoelectronic devices while maximizing performance (power and speed) and reliability. Gallium nitride material system has been used recently to fabricate microwave transistors with high-electron mobility (necessary for high-speed operation), high breakdown voltage (necessary for high power), and relatively high thermal conductivity (greater than GaAs, InP). Gallium nitride devices have also been investigated for light-emitting diodes for solid-state illumination as well as for medical and environmental laser applications.
Because GaN devices offer high current density and high voltage operation, they exhibit larger total power losses due to parasitic resistances and the inefficiency inherent to the amplification process. Although bulk GaN has low defect density, its applications in high power electronics and high-brightness light-emitting diodes is still limited by high temperature resulting from high power dissipation. These devices still require better thermal properties than GaN can offer. Gallium nitride thermal conductivity (1.3-2.25 W/Kcm) is lower than that of silicon carbide (3.5 W/Kcm).
In all of these applications, heat removal has conventionally been accomplished by placing the electronic device, optical device or integrated circuit on top of a heat sink [5]. A heat sink is a substrate or device for the absorption or dissipation of unwanted heat (as from a process or an electronic device). Most often, the heat sinks are copper blocks attached to a water-cooling system, aluminum fins, or a micro-channel cooler. Synthetic diamond heat sinks are being actively investigated because of diamond's superior thermal properties in respect to all other materials in nature [5]. Conventional heat removal systems for transistors and light-emitting devices (based on bonding and attaching devices to heat sinks) are typically large in comparison with the heat source in the electronic chip or individual device and as described next offer limited thermal performance improvement.
The efficiency of heat extraction from very small heat sources (such as a channel in a high-electron mobility transistor) critically depends on the ability to spread the heat flow to a larger area, because majority of the temperature drop occurs in the immediate proximity of three-dimensional heat source. A heat spreader (or heat sink) of high thermal conductivity is most efficient when the distance between the heat source and the heat spreader is smaller than the lateral dimension of the heat source. The heat sources in electronic devices are typically several micrometers wide, while the wafers on which these devices are grown are at least 100 um thick. This means that when wafers with such devices are mounted on heat sinks, this above-mentioned efficiency condition is generally not met. One exception to this structure is the case when the devices are mounted upside down on heat sinks, but this approach increases the manufacturing process complexity and is used only in limited number of situations.
It is clear therefore that there is a need in the industry to provide means for high-efficient thermal management of high-power devices by providing means to integrate electronic devices with very highly conductive materials, such as, synthetic diamond.
This invention discloses such devices and processes.